In order to transmit high quality and high capacity of data under limited radio resources in wireless communication systems, a variety of transmission methods have been developed.
A closed-loop system uses a feedback channel from a user equipment to a base station in order to transfer feedback information. In the closed-loop system, after receiving feedback information from the user equipment on the feedback channel, the base station maximizes the performance by adjusting several system parameter, such as power level and transmission format, based on the feedback information. For example, adaptive modulation and coding (AMC) scheme increase link performance by adjusting modulation and coding scheme (MCS) on the feedback information. When channel condition is good, the data rate is increased and when channel condition is bad, the data rate is lowered. Thus, efficient transmission can be supported and, therefore, an average transmission rate can be increased.
An example of the feedback information is channel quality indicator (CQI). In general, CQI is measured by a user equipment and is feedback through uplink channel. The base station can assign the most appropriate resources to the user equipment based on the CQI.
For example, it is assumed that 5 bits (25=32 level) are used for CQI transmission. CQI values between −10 dB(decibel) and 22 dB can be quantized and represented every 1 dB distance. Or, CQI can be represented by one of 32 MCS levels. The 5 bits of CQI are channel-coded and then transmitted. If CQI is missed or distorted during transmission, AMC scheme can be much degraded. Accordingly, CQI needs to have high code rate to guarantee reliable transmission. But high code rate requires lots of radio resources.
In particular, in a multi-carrier system based on Orthogonal Frequency Division Multiplexing (OFDM), CQI may differ per on frequency (sub-carrier) basis. To acquire perfect channel information, it is needed that CQIs for every sub-carrier are measured and reported. But this may cause signaling overhead and low data rate.
Accordingly, there is a need for a method of efficiently transmitting CQI.